what are the symptoms of a dying diff?

my old girl (91 Z71) is about to roll over into 300,000 KM, and she's still going very strong. I do get the feeling that the diff will need rebuilding
shortly.
what are the symptoms of a dying diff? the truck drives smoothly, no grinding or anything obvious. But I do get the tell-tale "clunk" on shifts from drive to reverse and back. I also sometimes get the "clunk clunk" that dead u-joints give on slowing down and then speeding up (you know, pulsing the throttle and "clunk clunk clunk")
The u-joints are clean, lubed and tight, so its not them. I'm convinced its the diff....somebody told me to drive the truck until the diff starts grinding badly, then rebuild. I just hate to be in the middle of nowhere during hunting season when that happens!!!
ideas?
b
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It Blows!!! Had an 88 S-10 w/ 4wheel drive blow on the highway... It made a crunch sound, then about 1/10 mi later another with a little jerk (enought to get over in 4 lanes), and then it blew a hole in the back and blew parts. It flamed a little but went out. After cooling a little I put it in 4 wheel drive and drove to a friends, then home. On the way from the friends to home he followed me and it was still blowing parts onto him (bearing balls, etc). We were concerned that it could have seized going home cause the oil is gone when it blows. Got another at the junk yard for $35 + $10 core and a couple hours to replace it. Elliott
LowDownZ28 wrote:

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could be a worn yoke or worn r&p gears.After a lot of miles the clearances get real loose hence your "cluncking" sound.
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